This invention relates to a high-strength curable composition for a fiber-reinforced resin. More particularly, it relates to a curable composition for a fiber-reinforced resin which is suitable for molding of structural parts for automobiles, heavy vehicles, and the like in which light weight and fatigue resistance are important.
Significant increases in the fuel efficiency and driving comfort of automobiles have been obtained by improvements in the design of body shape and internal combustion engines. Still further improvements are being achieved by the use of plastic parts, which are superior to metallic parts from the standpoints of lightness and workability. Plastic parts are being used increasingly for various structural parts such as drive shafts, steering wheels, bumpers, brackets, and springs.
In the past, many of these structural parts were made of steel. At present, some of these parts are made of lightweight alloys in order to achieve decreases in weight. However, the performance of lightweight alloys is still inadequate in certain respects, and for this reason, there is now an interest in fiber-reinforced resins which are lighter than alloys, do not rust, and have excellent mechanical properties.
For example, Japanese Published Patent Application No. 58-28454 discloses an FRP leaf spring for automobiles which has excellent fatigue resistance. The spring is manufactured by reacting a bisphenol-type epoxy resin and an unsaturated-monobasic acid in amounts such that there is approximately one equivalent of epoxy groups per each equivalent of the unsaturated-monobasic acid to obtain a vinyl ester resin, dissolving this resin in a polymerizable vinyl monomer to obtain a resin composition, impregnating fiber reinforcement with the resin composition, and then molding and curing.
In addition, "Special Steels" (Vol 34, No. 8, Aug., 1985, pages 51-55, in Japanese) contains an article which briefly explains new types of leaf springs. In the article, epoxy resins and vinyl ester resins are mentioned as suitable examples of a matrix resin. However, as stated therein, there is a need for resins which have a higher heat resistance, a lower viscosity, a longer pot life, a shorter curing time, and a lower cost than these two types of resins.
Presently-known fiber-reinforced resins include unsaturated polyester resins, epoxy resins, vinyl ester resins (also referred to as epoxy acrylate resins), urethane acrylate resins, polyamide resins, phenol resins, furan resins, and the like which are reinforced with fibers such as glass fibers, carbon fibers, polyamide fibers, and alumina fibers. Among these, polyamide resins, phenol resins, and furan resins are expensive and excessively difficult to mold. Accordingly, unsaturated polyester resins, epoxy resins, vinyl ester resins, and urethane acrylate resins are more frequently used. In particular, for parts such as springs which require fatigue resistance, high strength, and high stiffness, it is common to use epoxy resins which have excellent heat resistance and can be strongly bonded to reinforcing materials. Unsaturated polyester resins, vinyl ester resins, and urethane acrylate resins are far superior to epoxy resins with respect to their viscosity, which is low and facilitates the impregnation of fibers, and with respect to their curing speed, which is faster. However, the adhesiveness of these resins to fibers and their heat resistance are inadequate, and therefore they have not attained the reliability of epoxy resins.